Detections of 21 cm absorption with a blind FAST survey at z ≤ 0.09

¹ Aix-Marseille Univ., CNRS, CNES, LAM, 13388 Marseille, France
e-mail: wenkai.hu@lam.fr
² National Astronomical Observatories, Chinese Academy of Sciences, Beijing, 100101 PR China
e-mail: wangyg@bao.ac.cn, xuelei@cosmology.bao.ac.cn
³ ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Mt Stromlo Rd, Australian Capital Territory, 2611 Australia
⁴ Key Laboratory of Cosmology and Astrophysics (Liaoning) & College of Sciences, Northeastern University, Shenyang, 110819 PR China
⁵ School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing, 100049 PR China
⁶ Institute of Astronomy and Astrophysics, Academia Sinica, Astronomy-Mathematics Building, No. 1, Sec. 4, Roosevelt Road, Taipei, 10617 Taiwan, PR China
⁷ Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 S George Street, Toronto, ON, M5S 3H8 Canada
⁸ Canadian Institute for Advanced Research, 180 Dundas St West, Toronto, ON, M5G 1Z8 Canada
⁹ Dunlap Institute for Astronomy and Astrophysics, University of Toronto, 50 St George Street, Toronto, ON, M5S 3H4 Canada
¹⁰ Perimeter Institute of Theoretical Physics, 31 Caroline Street North, Waterloo, ON, N2L 2Y5 Canada
¹¹ Research Center for Intelligent Computing Platforms, Zhejiang Laboratory, Hangzhou, 311100 PR China

Received: 24 November 2022
Accepted: 25 April 2023

Abstract

We present the early science results from a blind search of the extragalactic H I 21 cm absorption lines at z ≤ 0.09 with the drift-scan observation of the Five-hundred-meter Aperture Spherical radio Telescope (FAST). We carried out the search using the data collected over 643.8 h by the ongoing Commensal Radio Astronomy FasT Survey (CRAFTS), which spans a sky area of 3155 deg² (∼81% of CRAFTS sky coverage up to January 2022) and covers 44827 radio sources with a flux density greater than 12 mJy. Due to the radio frequency interference (RFI), only the relatively clean data in the frequency range of 1.3–1.45 GHz are used in the present work. Under the assumption of T_s/c_f = 100 K, the total completeness-corrected comoving absorption path length spanned by our data and sensitive to Damped Lyman α Absorbers (DLAs; N_H I ⩾ 2 × 10²⁰ cm⁻²) is ΔX^inv = 8.33 × 10³ (Δz^inv = 7.81 × 10³) for intervening absorption. For associated absorption, the corresponding value is ΔX^asc = 1.28 × 10¹ (Δz^asc = 1.19 × 10¹). At each time point of the drift scan, a matched-filtering approach is used to search H I absorbers. Combining the information of observation mode and the distribution of the beams that detect the same candidates, spurious absorbers are successfully excluded. Three known H I absorbers (UGC 00613, 3C 293, and 4C +27.14) and two new H I absorbers (towards the direction of NVSS J231240–052547 and NVSS J053118+315412) are detected blindly. We fitted the H I profiles with multi-component Gaussian functions and calculated the redshift (0.063, 0.066), width, flux density, optical depth, and H I column densities for each absorption. Our results demonstrate the power of FAST in blindly searching H I absorbers. For absorption towards NVSS J231240–052547, the optical counterparts are faint and currently lack existing spectra. The most likely interpretation is that a radio-loud active galactic nucleus (AGN) is faint in the optical as the background source, with a faint optical absorber in between. NVSS J053118+315412 exhibits an associated absorption with a complex profile, which may suggest unsettled gas structures or gas accretion onto the supermassive black hole (SMBH). The expanding collection of blind radio detections in the ongoing CRAFTS survey offers a valuable opportunity to study AGNs, associated interstellar medium (ISM) interaction, and intervening absorbers optically without overwhelming quasi-stellar object (QSO) background light.